Speaker telephones have several definite advantages over the conventional telephone instruments. For example, the hands of the user of the speaker phone are free to perform functions other than holding a telephone handset. Also, the speaker phone enables a plurality of parties to simultaneously fully participate in telephone conversations over the single speaker phone.
Unfortunately, the prior art, presently available speaker phones suffer severe shortcomings caused by the unavoidable accoustical and electrical coupling between the microphones and the loud-speakers of the speaker phones. When incoming, received voice signals are amplified to drive the loud-speaker, the accoustical output of the loud-speaker is sensed by the microphone and returned to the telephone line as positive accoustical feedback, causing oscillations or "howling" at the receiving loud-speaker. In addition, the sensed signal is transmitted to the distant party, causing an unpleasant "echo" that simulates talking in a barrel.
The signals that originate at the microphone for transmission to the distant party over the telephone lines are at least partially electrically coupled back to the loud-speaker; and therefore, then also accoustically coupled back to the microphone. This feedback also may cause oscillation with the consequent noise and distortion of the outgoing signal.
The prior art uses many electrical systems to attempt to overcome the problems caused by the accoustical coupling of the loud-speaker and the microphone. The different systems are not always used exclusively.
Some of the more widely used systems are "cancellation", "switching" (i.e., the speakers and microphones are switched on and off separately and exclusively, whereby when the speaker is used, the microphone is "dead" and vice versa), and by "gain control"; i.e., by reducing the gain at the receiving end and the emitting end to prevent the troublesome and interferring oscillations.
Examples of the prior art attempted solutions are discussed and explained, for example, in U.S. Pat. Nos. 3,889,058 ("switching"), 3,975,588 (discussion of "cancellation"), 3,922,488 ("cancellation", "gain control " and "switching") and 4,052,562 ("switching").
Each of the prior art attempts at finding a solution to the "accoustical feedback" problem has certain shortcomings. The use of "cancellation" has not proved successful, since complete cancellation is not possible, because of such things as varying line impedance.
The "switching" solution prevents "simultaneous" conversation; and therefore, is inherently unnatural.
"Gain control" attempts have largely floundered, because equipment for accurately determining the source or direction of the signals; i.e., are the signals "incoming" or "outgoing", have been lacking. Therefore, in the prior art systems, either the gain was muted at the wrong time, or wasn't reduced when reduction was necessary.
Accordingly, an object of the present invention is to provide speaker telephones which include circuitry that successfully and economically eliminate problems arising from the accoustical and/or electrical coupling of the microphone and the loud-speaker.
A related object of the present invention is to provide circuitry for determining the "direction" of signals in duplex equipment, such as speaker telephones.
Another and related object of the present invention is to provide circuitry to cancel signals originating in the transmitting transducer (microphone) prior to the arrival of such signals at the receiving transducer (loudspeaker) without interfering with the transmission of those signals over the telephone lines.
Another object of the present invention is to keep the microphone and loud-speaker amplifiers operative, but to reduce the signal output of the microphone amplifier means during the receive mode and to reduce the signal output of the loud-speaker amplifier means during the transmit mode, with the modes determined by the direction determining means.
Still another object of the present invention is provide signal level detector means for use in controlling the signal level reducing means.
Another object of the present invention is to provide telephone loud-speaker systems effectively using cancellation, as well as shunt means, for controlling the output of the loud-speaker and the microphone suppliers, responsive to the direction of the signals and to the level of the signals with the circuitry powered from the telephone lines.
Yet a further related object of the present invention is to control the amplifier shunts with protective timing circuits to prevent and reduce noises, such as clicking noises.